The use of water-swellable, generally water-insoluble, absorbent materials in disposable absorbent personal care products is known. Such absorbent materials are generally employed, for example, in absorbent products such as diapers, training pants, adult incontinence products, and feminine care products in order to increase the absorbent capacity of such products while reducing their overall bulk. Such absorbent materials are generally present in absorbent products in a fibrous matrix, such as a matrix of wood pulp fluff. A matrix of wood pulp fluff generally has an absorbent capacity of about 6 grams of liquid per gram of fluff. Some of these materials having an especially high absorbent capacity are commonly known as superabsorbents or superabsorbent polymers (SAPs). Water-swellable, water-insoluble absorbent materials have an absorbent capacity of at least about 10, desirably of about 20, and often of up to about 1000 times or more their weight in water. Incorporation of such absorbent materials in personal care products can reduce the overall bulk while increasing the absorbent capacity of such products.
A wide variety of materials have been described for use as water-swellable, water-insoluble absorbent materials in personal care products. Such materials include, but are not limited to, natural-based materials such as agar, pectin, gums, carboxyalkyl starch, and carboxyalkyl cellulose, as well as synthetic materials such as polyacrylates, polyacrylamides, and hydrolyzed polyacrylonitrile. While such natural-based absorbent materials are known for use in personal care products, they have not gained wide usage in such products, at least in part because their absorbent properties are generally considered inferior compared to that of the synthetic absorbent materials, such as the sodium polyacrylates. Specifically, many of the natural-based materials tend to form soft, gelatinous masses when swollen with a liquid. When employed in absorbent products, the presence of such soft gelatinous masses tends to prevent the transport of liquid within the fibrous matrix in which the absorbent materials are incorporated. This phenomenon is known as gel blocking. Once gel blocking occurs, the product cannot efficiently absorb subsequent insults of liquid, and the product tends to leak. Further, many of the natural-based materials exhibit poor absorption properties, particularly when subjected to external pressures.
In contrast, synthetic absorbent materials are often capable of absorbing large quantities of liquid while maintaining a generally stiff, non-mucilaginous character. Accordingly, synthetic absorbent materials can more readily be incorporated into absorbent products while minimizing the likelihood of gel blocking.
One property of many available water-swellable, water-insoluble materials, especially superabsorbent materials, is that such materials typically very rapidly absorb the liquid that comes into contact with the superabsorbent material. While such quick absorbency of the liquid may be desirable in many applications, there are certain applications in which it is not desirable. For example, in an absorbent structure that is insulted with a liquid at only a very localized location, it would generally be desirable to have the liquid distributed throughout the entire volume of the absorbent structure so that the absorbent capacity of the entire absorbent structure is utilized. However, if the superabsorbent material located near the localized insult location absorbs the liquid very quickly, this superabsorbent material may swell and restrict the flow of the liquid throughout the rest of the absorbent structure, possibly resulting in the liquid leaking out of the absorbent structure in the area of the localized insult location. Accordingly, it is often desirable to employ a superabsorbent material in the localized insult location that actually absorbs the liquid at a reduced rate. This allows the liquid to be distributed throughout the absorbent structure first and then be subsequently absorbed by the superabsorbent material.
Several methods are used to slow the liquid absorbing rate of a relatively fast absorbing superabsorbent material. For example, it is possible to coat the fast absorbing superabsorbent material with a material that is less absorbent than the superabsorbent and/or more hydrophobic than the superabsorbent. Such coating materials tend to temporarily shield the underlying superabsorbent material from any liquid and, thus, delay the absorption of the liquid by the superabsorbent material. However, such coating materials often reduce the overall liquid capacity of the superabsorbent material, add to the expense and complexity of preparing the superabsorbent material, and may negatively affect other liquid handling properties of the superabsorbent material.
Commercially available superabsorbents are generally in a substantially neutralized or salt form. This is because in many cases water-swellable, water-insoluble polymer that are polyelectrolytes have a greater absorbent capacity than related acids and bases. However, such superabsorbents absorb liquid relatively quickly, leading to the above discussed problems. Thus, in many cases, when an acidic or basic water-swellable, water-insoluble polymer, substantially in its free acid or free base form, respectively, is mixed with a basic second material or an acidic second material, respectively, the resulting absorbent composition will exhibit both a fairly high capacity for liquid absorption as well as a relatively slow liquid absorbing rate. This is believed to be because, as the mixture is placed in an aqueous solution, the acidic or basic water-swellable, water-insoluble polymer, substantially in its free acid or free base form, respectively, reacts with the basic second material or the acidic second material, respectively, and the chemical equilibrium is in favor of converting the acidic or basic water-swellable, water-insoluble polymer from its free acid or free base form, respectively, to its respective salt form. As such, the mixture comprising the converted water-swellable, water-insoluble polymer will now exhibit a relatively high capacity for liquid absorption. However, because the conversion of the water-swellable, water-insoluble polymer, from its free acid or free base form, respectively, to its respective salt form is a relatively slow process of ionization and ion diffusion into the polymer, the water-swellable, water-insoluble polymer will also exhibit a relatively slow liquid absorbing rate. In addition, the conversion of the water-swellable, water-insoluble polymer from its free acid or free base form to its respective salt form in an electrolyte-containing solution, such as an aqueous sodium chloride solution, has a substantial desalting effect on the electrolyte-containing solution, thereby improving the liquid-absorbing performance of the mixture by alleviating the salt-poisoning effect.
WO 98/24832 discloses an absorbent composition that includes a polymeric absorbent material and a second material. The two components are mixed and used in an absorbent article, desirably in conjunction with a fibrous matrix. The second material can be provided in the form of particles, flakes, fibers, films, and nonwoven structures or the two components can be provided as a bi-component fiber. This composition has disadvantages in terms of handling properties and structural integrity.
A single material or polymer comprising both acidic and basic functional groups within its molecular structure will not exhibit the desired absorbent properties described above. This is believed to be because such acidic and basic functional groups within a single molecular structure will typically react with each other, possibly resulting in an over-crosslinked polymer structure. As such, it generally is not desirable to prepare an absorbent composition by preparing a copolymer from acidic and basic monomers or by preparing a molecular level dispersion, such as in an aqueous solution, of water-soluble acidic and basic materials since during such copolymerization or molecular level dispersion the acidic and basic materials will typically react with each other and crosslink.
Accordingly, it is an object of the present invention to provide a fibrous matrix containing fibers that release an activating agent upon contact with an insult liquid, wherein the activating agent causes an absorbent material to form a superabsorbent polymer.
It is further an object of the present invention to provide a fibrous matrix containing fibers that release an activating agent upon contact with an insult liquid, wherein the activating agent causes an absorbent material to increase in its water absorbing capacity.
It is also an object of the present invention to provide an absorbent structure containing a polymer that is not a superabsorbent polymer, but that forms a superabsorbent polymer in situ upon contact of the absorbent structure with a liquid to be absorbed.
It is also an object of the invention to provide an absorbent structure containing a water-swellable, water insoluble polymer that experiences an increase in water absorbing capacity upon contact of the absorbent structure with a liquid to be absorbed.
A further object of the present invention is to provide methods for making the foregoing structures and matrices.
A further object of the present invention is to provide absorbent articles containing the foregoing structures and matrices.
Yet another object of the present invention is to provide a method for forming superabsorbent polymers in situ.